Just one fizzy drink a day raises the risk of developing diabetes by a fifth, scientists claim.

Each subsequent calorie-laden can increases the chance of the type 2 form of the disease by the same amount, according to findings.

Researchers say the trend is not necessarily caused by obesity brought on by the high sugar content of the beverage.

Dr Dora Romaguera, who led the study, said: ‘Given the increase in sweet beverage consumption in Europe, clear messages on its deleterious effect on health should be given to the population.’

Writing in the diabetes journal Diabetologia, she added: ‘One of the problems with soft drinks is although they cause diabetes because of the high content of sugar, you do not feel you have eaten anything – so you go on consuming them.

‘Fizzy drinks should be seen as a treat you have once every couple of weeks – not as a substitute for water to be had every time you feel thirsty.’

Researchers at Imperial College London studied 12,403 patients with type 2 diabetes and a random group of 16,154 participants from eight countries, including Britain. Dr Matthew Hobbs, of Diabetes UK, said the findings were not ‘definitive’ and suggested maintaining a healthy weight is the most effective way to prevent type 2 diabetes.

Gavin Partington, head of the British Soft Drinks Association, said the study had not taken account of other factors such as weight and family history. Soft drinks are safe to consume in moderation, he added.